Astronomers hunt for exomoons at 'the hairy edge'

For NASA's Kepler mission, finding Earth-like planets is as difficult as trying to spot a firefly hanging out near a spotlight from thousands of miles away, Astronomer David Kipping of Harvard is going one step further: he's trying to find specks of dust floating around the firefly that's flying around the spotlight.

“For moons the effects are likely to be not just small, but minute—right on the hairy edge of what Kepler can do” -Greg Laughlin

As one of the first astronomers to seriously hunt for moons that orbit exoplanets, or planets that orbit stars beyond our sun, Kipping has no illusions about the fact that he's taken on an unusually challenging task. "Finding moons is pushing the limit," he said. Or, as exoplanet scientist Greg Laughlin of the University of California, Santa Cruz said, finding moons is "right on the hairy edge of what Kepler can do."

Kipping, whose research is partially funded by a NASA Carl Sagan Fellowship, explains that the hunt for these so-called "exomoons" could have a major scientific payoff. "There may be lots of habitable moons in the galaxy, like the world Pandora in the movie Avatar," he said. "In fact, the nearest habitable world may not be a planet, but Jupiter's moon Europa, so it makes sense to look for Earth-like moons."

Kipping also points out that our own moon has been instrumental in the evolution of life on Earth. "The moon stabilizes Earth's axis and contributes to the ocean tides. It keeps the seasons from being erratic and extreme." Moons may be essential in maintaining planetary environments that are friendly to life as we know it, so it's important to develop techniques to identify them.

Planets in the habitable zone that are too large to have terrestrial surfaces may have smaller moons that could be Earth-like.

The search for exomoons can also help astronomers understand the unusual moons in our own solar system. "In the solar system, our moon is kind of a freak," Kipping said. "It's very large compared to the size of Earth. We want to know if these kinds of moons are common in the galaxy or not." More knowledge about the galactic distribution of moons in the galaxy could help refine theories of how solar systems form and determine whether such "freaky" cases as Triton, which orbits Neptune backwards and is thought to be a former dwarf planet that was captured by Neptune's gravity, are as rare in other solar systems as they are in ours.

Kipping began his hunt for exoplanets in Fall 2011, eventually forming the Hunt for Exo-moons with Kepler (HEK) with some colleagues, including the amateur astronomer Allan Schmitt. Schmitt, using the public repository of Kepler data at planethunters.org, had found some light curves that suggested the presence of moons orbiting the planets Kepler had found.

Together, they set out to shake down the mission's vast data sets for evidence of exomoon observations. "Kepler is designed to find planets down to one Earth-radius, so it makes sense to use it to also try to find moons that are the size of Earth," he said. "We did research into the topic and found that Kepler can indeed detect habitable moons."

Exomoons are far too tiny to be seen directly by any telescope yet invented, but Kipping says that they do "leave lots of clues that we can piece together." Kepler finds planets by observing the dip in light that happens when an exoplanet orbits directly between its host star and the telescope. This "transit" happens every time a planet orbits its star.

The gravity of a moon orbiting a planet may cause the planet to speed up or slow down, causing minute variations in the timing of its transits each time it travels in front of the star. And when the moon itself passes in front of the star, scientists can see the dip in light it causes, giving them further clues. Using these methods, Kipping and his colleagues can determine the size of a moon and its distance from its host planet.

The challenge for the HEK team is finding objects in the Kepler data archives that are a good place to start hunting for exoplanets. "We need to find planets that are dynamically capable of having moons, based on what we know about their size and orbit," Kipping said. "A hot Jupiter, for instance, probably can't have an Earth-size moon because it's way too close to its star."

Furthermore, because the signal exomoons produce is so incredibly tiny, Kipping's team needs to identify Kepler planets that have "pure, clean data, and a good signal-to-noise ratio."

From there, the planetary information gets fed into a pair of servers that Kipping has purchased for HEK with the help of Sagan Fellowship funds and donors from the science crowd-funding website petridish.org. "A lot of the resources for this project are dedicated to server time," Kipping said. "We have to analyze the light curve and timing variations to make sure that what we're looking at is a moon and not a multiple-exoplanet system." Earlier this year, HEK discovered a new planet that had been previously overlooked in the Kepler data, nicknamed Kepler-42 c, by looking at transit-timing variations.

Following Kipping's lead, planetary scientists have begun theorizing about the different kinds of conditions that could support life as we know it on distant exomoons. And as Kepler continues to observe stars, the HEK team will likely have more information on planets that are farther away from their stars and can support moons the size of Earth. "We need at least 1,000 days of observations to start seeing exoplanets that could have exomoons," Kipping said. "And Kepler has been observing for just about 1,000 days now, so we should start getting more and more good candidates to work with."

"What's fun about this is that it's a shot in the dark," Kipping said. "It's invigorating. Are moons common in the galaxy? Could there be as many or more Earth-like moons than Earth-like planets? No one knows about exomoons, which makes them a lot of fun to study ."

NASA's Exoplanet Science Institute at the California Institute of Technology, Pasadena, Calif., manages the Carl Sagan Fellowship and time allocation on the Keck telescope for NASA. NASA’s Jet Propulsion Laboratory, also in Pasadena, manages NASA's Exoplanet Exploration program office. More information about exoplanets and NASA's planet-finding program is at http://planetquest.jpl.nasa.gov.